Display empty schemas on the sidebar (#621)

* Display empty schemas on the sidebar
* Add schemas fetching test
* Skip pg_temp schemas
* Exclude pg_temp tables from other queries

pkg/statements/sql/schemas.sql

@@ -2,5 +2,8 @@ SELECT
   schema_name
 FROM
   information_schema.schemata
+WHERE
+  schema_name NOT IN ('information_schema', 'pg_catalog')
+  AND schema_name !~ '^pg_(toast|temp)'
 ORDER BY
   schema_name ASC